CN111328310B

CN111328310B - Method of calibrating printer, printing system, and computer-readable storage medium

Info

Publication number: CN111328310B
Application number: CN201780096717.2A
Authority: CN
Inventors: X·昆特罗鲁伊斯; R·罗德里格斯阿隆索; M·索拉诺帕拉罗
Original assignee: Hewlett Packard Development Co LP
Current assignee: Hewlett Packard Development Co LP
Priority date: 2017-11-13
Filing date: 2017-11-13
Publication date: 2022-01-04
Anticipated expiration: 2037-11-13
Also published as: US11135834B1; EP3691909A4; US20210283903A1; WO2019094046A1; EP3691909B1; CN111328310A; EP3691909A1

Abstract

Certain examples describe methods of calibrating a printer and a printing system. The interference pattern is printed on the print medium in the form of a printed calibration image. Data representing an interference pattern printed on a printing medium is detected by using an optical sensor. The printer is calibrated based on the captured data. The interference pattern is based on a waveform that varies in amplitude along an axis perpendicular to the calibrated printing axis and has a repeating set of a plurality of patterns based on the waveform with varying pattern spacing across the calibrated printing axis.

Description

Method of calibrating printer, printing system, and computer-readable storage medium

Technical Field

The present disclosure relates generally to printer calibration.

Background

In a printing system, a pattern may be printed to help calibrate the printer. For example, a scanning inkjet printer may include an inkjet pen mounted on a movable carriage. Each pen may contain a printhead having a plurality of ink ejection nozzles. During printing, the carriage may move across a print medium (such as a sheet of paper) as the nozzles eject ink drops. The timing of the ejection of the ink droplets can be controlled to accurately place the ink droplets at the desired locations. In such inkjet printers, a pattern may be printed to calibrate the alignment of the print head so that ink may be deposited in the correct location. Misalignment may occur, for example, due to movement of the printhead in the carriage mounting, mechanical misalignment of the carriage, and/or misalignment in the media transport system.

Disclosure of Invention

According to an aspect of the present disclosure, there is provided a method of calibrating a printer, including: printing an interference pattern on a printing medium; capturing data representing an interference pattern printed on a print medium by using an optical sensor; calibrating the printer based on the captured data, wherein the interference pattern is based on a waveform that varies in amplitude along an axis perpendicular to the calibrated printing axis, and wherein the interference pattern comprises a repeating set of a plurality of patterns based on the waveform, the repeating set having varying pattern intervals over the calibrated printing axis.

According to another aspect of the present disclosure, there is provided a printing system including: a print head for printing an image on a print medium; an optical sensor for capturing data from the printed image; a memory for storing a definition of a printed calibration image comprising interference patterns of varying spacing on a print axis, the interference patterns comprising waveforms of varying amplitude along an axis perpendicular to the print axis; and a calibration controller for calibrating the printing system on the print axis, the calibration controller comprising a processor for: obtaining a definition of a print calibration image from a memory; instructing to print a print calibration image using a print head; receiving captured data from an optical sensor associated with printing a calibration image; and processing the captured data to determine calibration parameters for the printing system.

According to another aspect of the present disclosure, there is provided a non-transitory computer readable storage medium comprising a set of computer readable instructions stored thereon which, when executed by a processor of a printing system, cause the processor to: instructing the printing system to print a printed calibration image comprising interference patterns of varying spacing on a printing axis, the interference patterns comprising waveforms of varying amplitude along an axis perpendicular to the printing axis; receiving captured data from an optical sensor associated with printing a calibration image; and calibrating the printing system based on the captured data.

Drawings

Various features of the disclosure will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, the features of the disclosure, in which:

FIG. 1 is a schematic diagram of a printing system according to an example;

FIG. 2 is a schematic diagram showing an interference pattern of a pen crossing print axes according to an example;

FIG. 3 is a graph representing captured data according to an example;

FIG. 4 is a schematic diagram showing an interference pattern of a pen print spindle according to an example;

FIG. 5 is a flow chart illustrating a method of calibrating a printer according to one example;

FIG. 6 is a schematic diagram showing two interference pattern waveforms according to an example; and

fig. 7 is a schematic diagram representing a non-transitory computer-readable storage medium according to an example.

Detailed Description

Certain examples described herein provide a form of interference pattern that enables calibration of a printer within a printing system. This specially shaped pattern is very effective for aliasing. It may also accommodate a nozzle miss and/or malfunction. It allows the signal-to-noise ratio to be improved without additional filtering of the captured data. It can be used to calibrate the alignment of the printhead and/or the media transport system.

Fig. 1 shows a printing system 100 according to an example. Printing system 100 may comprise a two-dimensional or three-dimensional printing system. The printing system includes a printhead 110 for printing an image on a print medium 120. In a three-dimensional printing system, a printer may deposit an agent, such as a modeling agent, onto a bed of build material, such as a bed of powdered build material.

In the example of fig. 1, the printhead 110 moves across the width of the print medium in direction 130, while the print medium 120 advances below the printhead 110 in direction 140. The print media may comprise a sheet or continuous web of media. Any form of print media may be used, including paper, cardboard (i.e., corrugated media), fabric, and polymer film. In a three-dimensional printing system, the print media may comprise a sheet of media (such as paper) or a bed of build material.

The print head 110 may include a plurality of nozzles. The nozzles may be aligned in one or more columns along the length of the printhead (e.g., in a direction parallel to direction 140 in fig. 1). For example, the print head 110 may comprise an inkjet print head. Ink or a modeling agent may be ejected through nozzles of the printhead. The print head may be a thermal or piezoelectric print head. In some cases, the printhead may be mounted within a carriage that moves across print medium 120 in direction 130. Taking ink as an example here, printhead 110 may alternatively deposit other printing fluids, such as pre-print agents and post-print agents (such as varnish, gloss, under-treatment).

In other examples, the printhead may form part of a page wide array printer. In these examples, there may be no movement in direction 130; rather, multiple printheads may extend across the width of the print medium 120. In this case, the position of the printed image on the print medium 120 in the direction 130 may be controlled by activating different nozzles along the width of the page wide array.

The printing system 100 of fig. 1 also includes a calibration controller 150. Calibration controller 150 may include a printed circuit board and/or an integrated circuit. Calibration controller 150 may be located within a printer of printing system 100 or may comprise a computer system electronically coupled to such a printer. The calibration controller 150 may form part of a control subsystem that is electronically coupled to a broader control system, for example, may be coupled to other printed circuit boards via a system bus. Calibration controller 150 may include a processor in the form of a central processing unit, microprocessor, or system-on-a-chip device. In FIG. 1, calibration controller 150 is electronically coupled to memory 160. The memory 160 may include volatile memory and/or non-volatile memory. In some cases, memory 160 may include non-volatile memory to store instructions to calibrate controller 150 and configuration data for the printing system. Data may be transferred from the non-volatile memory to the volatile memory during operation, where the processor of the calibration controller 150 may access the data and instructions stored in the volatile memory. Volatile memory can include any form of Random Access Memory (RAM), and non-volatile memory can include solid-state memory, magnetic storage devices, and/or read-only memory (ROM), among others. The instructions stored in the memory 160 may be loaded and executed by the processor of the calibration controller 150 to implement the functions described herein.

In the example of fig. 1, the memory 160 stores a definition 170 of a print calibration image. In this example, the printed calibration image includes interference patterns that vary in spacing on the print axis. The print axis may include one of

directions

130 or 140. Multiple print axes may be calibrated, with each print axis having a different print calibration image. In one case, the definition 170 of the print calibration image may include the image to be printed, for example, in the form of a bitmap or the like. In another case, the definition 170 of the print calibration image may include a definition of the function used to generate the print calibration image. In this case, the definition 170 of the print calibration image may include program code and parameter values that control the printhead 110 to produce the print calibration image on the print medium. In any case, the calibration controller 150 is configured to obtain the definition 170 of the print calibration image from the memory 160 and instruct the print head 110 to print the print calibration image. In the example of fig. 1, this involves printhead 110 moving across print medium 120 in direction 130. During printing of the printed calibration image, the print medium 120 may also advance in the direction 140, e.g., printing may include multiple scans across the print medium 120 in the direction 130 as the print medium 120 advances in the direction 140.

In fig. 1, the printing system 100 further includes an optical sensor 180. The optical sensor 180 is configured to capture data from a printed image. The optical sensor 180 may include a reflection sensor configured to measure an intensity of reflected light (e.g., an intensity of light originally emitted by the optical sensor and reflected from the print medium 120). In other examples, optical sensor 180 may capture light emitted by another component of printing system 100.

In the example of fig. 1, optical sensor 180 is also configured to scan across the width of print medium 120 in direction 130. For example, the optical sensor 180 may be mounted in the same movable carriage as the print head 110, or may be mounted in an independently movable carriage. In other examples, the optical sensor 180 may extend across the width of the print medium 120, such as in a page wide array printer, for example. If the optical sensor 180 is mounted behind the print head 110 in direction 140 as shown in fig. 1, then the scanning of the optical sensor 180 in direction 130 (or the readout of a static page-wide array sensor) may measure a particular swath or swath (swathe) of the printed calibration image previously printed by the print head 110. If an optical sensor is mounted across the width of the print medium, it may be configured to output data from multiple spatial locations across the print medium.

In some cases, optical sensor 180 comprises a line sensor, i.e., it outputs measured light intensity values for a given field of view in

directions

130 and 140. An example field of view may be between 1mm and 2mm in both directions. After scanning print medium 120, optical sensor 180 may output an array of light intensity values corresponding to different lateral positions across the width of print medium 120. The array may comprise a one-dimensional array of length n, where n is equal to the number of measurements corresponding to the number of spatial locations across the width of the print medium 120. For example, n may be equal to 1000.

In some examples described herein, the interference pattern of the printed calibration image includes a waveform that varies in amplitude along an axis perpendicular to the axis of printing being calibrated. Example waveforms are described below with reference to fig. 2, 4, and 6. For example, these patterns avoid firing (fire) of all nozzles of the printhead 110 at the same time, as compared to a straight line in the direction 140. They also reduce the effect of aliasing produced by the optical sensor 180. For example, when the optical sensor 180 scans a comparison pattern comprising a series of straight lines in the direction 140, there is some spatial aliasing as the lines enter and leave the field of view of the optical sensor 180. This may generate a noise signal, as described later with reference to fig. 3. By using a waveform that varies in amplitude along an axis perpendicular to the print axis being calibrated, the pattern itself provides inherent anti-aliasing when captured by the optical sensor 180. This reduces noise so that post-processing of the captured data (e.g., using a low pass or moving average filter) can be reduced or avoided.

In the example of fig. 1, calibration controller 150 receives captured data from optical sensor 180, where the captured data relates to measurements of previously printed calibration images. This captured data is then processed to determine calibration parameters for printing system 100. For example, the calibration controller 150 may be configured to detect extreme values, such as maxima or minima, within the captured data. The extremum may indicate a spatial location where reflectivity is maximized, e.g., where two spaced interference patterns are most closely aligned. This spatial position may be used to determine misalignment within printing system 100. This misalignment can then be corrected, for example, by configuring the offset, etc.

Fig. 2 shows a simplified example 200 of printing a calibration image 210. This may include printing a calibration image of a print axis parallel to a scan axis of the printing system (e.g., in fig. 1, a print axis parallel to direction 130). This print axis may be referred to as the "pen-over" (CP) axis, where the length of the printhead includes the "pen" (P) axis. The pen axis may comprise an axis along which the nozzles of the print head are aligned (e.g., the number of nozzles in the direction of the pen axis may be much larger than the number of nozzles on the pen intersecting axis). The pen cross axis may be parallel to the direction in which the carriage including the printhead moves, i.e., may correspond to a direction across the width of the print medium.

Fig. 2 shows three repeating

sets

220, 230 and 240 of patterns based on a common waveform. In fig. 2, the waveform is a sinusoidal waveform. In this example, each repeating set of patterns includes two patterns: a first pattern 250 and a second pattern 260. They are indicated in fig. 2 by solid and dashed lines. The patterns have different pitches in the pen-crossing direction. For example, in fig. 2, the spacing between the two patterns decreases towards the center of the printed calibration image. For clarity, a reduced number of patterns are shown in FIG. 2; in some implementations, there may be any number of repeating sets of patterns (e.g., 20 to 40 in one example) depending on the printer configuration. In fig. 2, both patterns extend along an axis parallel to the pen axis.

In some cases, the first pattern 250 of the set of repeating patterns may be printed with a first printhead. In this case, the second pattern 260 may be printed with the second print head. For example, the two printheads may form portions of a printer pen with different color inks. If the printheads are aligned, the two patterns should overlap (i.e., interfere) at the center of the print medium. If the printheads are misaligned, one of the other sets of patterns may overlap. For example, if the first printhead has been shifted in the pen-crossing direction (e.g., to the right in fig. 2), the pattern 220 may overlap instead of the pattern 230. In more complex cases, there may be more than two patterns corresponding to multiple printheads. However, analysis can generally be simplified by performing a pair-wise calibration between multiple printheads. In this case, the first print head may be considered to belong to the reference pen, and the second print head may be considered to belong to the measured pen.

In some cases, the first pattern 250 and the second pattern 260 may be printed by using the same print head. For example, in a bi-directional calibration, the first pattern 250 may be printed by a printhead traveling in the direction 130, and the second pattern 260 may be printed by the same printhead traveling in the opposite direction (e.g., on the return) across the width of the print medium. In another case, the printhead may have more than one die or column of nozzles. In this case, the first pattern 250 may be printed by a first die or column of nozzles within a printhead, and the second pattern 260 may be printed by a second die or column within the same printhead. In other cases, the first pattern 250 may be printed and then the print medium may be moved before the second pattern 260 is printed. In this case, the pattern may be used to determine the alignment of the media transport system. It will be appreciated that each repeating set may comprise more than two patterns, for example if more than two print heads or nozzle columns are calibrated, taking the example of a set of two patterns.

In some examples, the waveform is configured to: such that a field of view of an optical sensor configured to scan the printed calibration image in the cross-pen direction is a non-zero multiple of the waveform period. For example, if the field of view of the optical sensor is 1.5mm, the period of the waveform may be 0.375mm or 0.75 mm.

FIG. 3 is a graph 300 representing captured data according to an example. The x-axis represents, for example, a spatial dimension corresponding to a measurement location across the width of the print medium. The y-axis represents an optical measurement, such as a measurement of light intensity. In fig. 3, line 310 represents the data value output by the optical sensor. For example, these data values may comprise a one-dimensional array of light intensity values generated as the optical sensor scans across the printed calibration image 210 in fig. 2. The line 310 has a minimum 315 at which the first pattern 250 and the second pattern 260 interfere. By reading the x-axis value of the minimum 315, misalignment can be determined.

For example, a fully aligned printing system may print the printed calibration pattern 210 such that the minimum 315 occurs at a spatial location 500 of 1000, i.e., at the center of the image. A misaligned printing system may print the printed calibration pattern 210 such that the minima 315 occur at different spatial locations, for example, 400 out of 1000 indicating a-100 misalignment, or 600 out of 1000 indicating a +100 misalignment. The measured misalignment may then be used to calibrate future printing operations, e.g., in the above example, the print offset may be set to +100 or-100, respectively, to return the minimum value 315 to the center of the printed image. In some cases, a set of misalignments may be converted into a measurement based on a number of points at a particular resolution. For example, misalignment or error may be measured in the range of-5 to +5 dots at a resolution of 1200 Dots Per Inch (DPI). This may correspond to an original x-axis position range from-500 to 500 (e.g., an x-axis position range of [ -500, -400, -300, -200, -100, 0, 100, 200, 300, 400, 500 ]). If the minimum falls at the x-axis value of-100, this can be mapped to a-1 point error. In this case, the correction may involve printing an image offset by 1 dot. In some cases, sub-dot resolution may be possible, for example, a minimum of-125 may equal a 1.25 dot error. In other cases, misalignment values may be grouped into discrete bins, e.g., a minimum value of-125 may be grouped into a-1 point error group.

FIG. 3 also represents a line 320, which line 320 indicates an output that may be generated when the optical sensor scans a comparative printed calibration image that includes a series of straight lines aligned with the direction 140. It can be seen that the data represented by line 320 is noisy and has a spiky or saw-tooth shape. Such variations in the data are typically filtered before the extrema can be located (e.g., it may cause the signal processing function to find a local minimum rather than a global minimum). Such filtering may affect the location of extrema of the processed position fix, e.g., the moving average filter may shift the location of the minima, so that any correction has a small error. In contrast, line 310 represents the signal received from the optical sensor while scanning the waveform described herein; this waveform post-processing of the signal can be avoided.

In certain examples, the optical sensors described herein may measure diffuse reflectance from a surface of a print medium when illuminated by one or more Light Emitting Diode (LED) light sources. Such optical sensors may operate by projecting illumination onto the print medium at an angle. In this case, the light may strike the paper at the intersection of the optical axes of the central diffusely reflective imaging lens. The reflected illumination may then be imaged onto two detectors, such as two light-to-voltage converters. In some cases, using two detectors, a central detector may capture the diffuse reflected component of the illumination reflection, while an external detector may capture the specular reflected component of the reflection.

Fig. 4 shows another simplified example 400 of printing a calibration image 410. This may include printing a calibration image of a print axis that is perpendicular to a scan axis of the printing system (e.g., a print axis that is parallel to direction 140 in fig. 1). As described above, the printing shaft may be a pen shaft.

Fig. 4 shows two repeating

sets

420, 430 of patterns based on a common waveform. In fig. 4, the waveform is a sinusoidal waveform. In this example, each repeating set of patterns includes two patterns: a first pattern 450 and a second pattern 460. They are indicated by solid and dashed lines in fig. 4. Also, these patterns may be printed by different print heads. The patterns have different pitches in the pen direction. In fig. 4, this is achieved by rotating the spatial axis of the second pattern relative to the first pattern (the spatial axis is perpendicular to the amplitude axis). This results in the second pattern 460 being spaced above the first pattern 450 to the left of the center of the printed calibration image 410, and then the second pattern 460 being spaced below the first pattern 450 to the right of the center of the printed calibration image 410. In fig. 4, it can be seen that the spacing between the two patterns decreases towards the center of the printed calibration image 410. For clarity, a reduced number of patterns are shown in FIG. 4; in some implementations, there may be 10 to 30 repeating sets of patterns.

When using the printed calibration image 410 of fig. 4, the optical sensor can scan along the spatial axis of the first pattern 450, wherein the measured reflected signal is minimized when the two patterns interfere. The waveform may again be configured such that the field of view of the optical sensor (e.g. in the direction of the pen) is a multiple of the period of the waveform.

The data obtained from scanning the optical sensor of the printed calibration image 410 of fig. 4 may be similar to the line 310 shown in fig. 3. Although the pattern of fig. 4 differs from the pattern of fig. 2 on the print medium, the data signal may look similar when the optical sensor integrates reflection from the print medium.

The interference pattern shown in fig. 4 may reduce or avoid the adverse effects of missing and/or malfunctioning nozzles because not all nozzles participate in the printing of the pattern. In contrast, a relatively straight line pattern can be printed using a large fraction of the nozzles in the printhead (e.g., the nozzle columns on the pen shaft). In this comparison case, a missing and/or malfunctioning nozzle results in a reflectivity measurement that is higher than expected, which disrupts subsequent signal processing. Missing and/or malfunctioning nozzles may be in the reference pen or the pen being measured.

The interference patterns described herein also reduce the need for nozzles of the print head to be used in the calibration process. For example, the variation in the waveform means that the nozzles do not have to fire simultaneously at high frequencies. If the nozzles are not necessarily simultaneously at high frequencies, the printhead and nozzle life may be increased, peak current consumption may be reduced, and the quality of the ejected ink drops may be improved. For example, when using the pattern shown in fig. 2, a small subset of nozzles is firing anywhere across the width of the print medium (e.g., in direction 130). The subset is then changed (e.g., moved along direction 130) for a subsequent position. With a relatively straight pattern, many nozzles in a column on the pen shaft fire to print a line in a given pass of the printhead. Similarly, when the pattern shown in fig. 4 is used, there are no nozzles that fire continuously across the width of the print medium. In contrast, a rectilinear pattern typically has a collection of nozzles at least for a reference pattern that is continuously emitted at high frequency across the width of the print medium. In the comparative case, the nozzle strain (strain) may be reduced by printing multiple passes; however, the presently described examples reduce nozzle strain when a single pass is used. Furthermore, multiple passes are slow and may result in additional shifts occurring between passes.

FIG. 5 shows a method 500 of calibrating a printer according to an example. At block 510, an interference pattern is printed on a print medium. The interference pattern is based on a waveform that varies in amplitude along an axis perpendicular to the axis of the print being calibrated, as shown, for example, in fig. 2 and 4. The interference pattern comprises a repeating set of waveform-based patterns, represented, for example, by first and

second patterns

250 and 260 in fig. 2 and first and

second patterns

450 and 460 in fig. 4. The repeating sets of patterns have different pattern pitches on the print axis. For example, at the beginning of a repeating set, a first pattern may lead a second pattern by an amount m. Across the repeating set, m may decrease until at the end of the set, the second pattern leads the first pattern by an amount m. At block 520, data representing an interference pattern printed on a print medium is captured using an optical sensor. This may include obtaining data as shown by line 310 in fig. 3. At block 530, the printer is calibrated based on the captured data. For example, an extremum may be detected in the captured data, and the spatial location corresponding to the extremum may be used to calibrate the printer, e.g., setting an offset to align the print.

In some cases, block 510 may be repeated for each of the pen and pen cross-print axes, where the pen axis corresponds to the length axis of the print head with the nozzles aligned. The printer may then be calibrated on the pen axis and the pen cross axis. Calibration on the pen shaft may include setting a nozzle offset to shift the firing pattern axially up or down with respect to the length of the print head, e.g. the firing pattern may be shifted by a-nozzle, where a is proportional to the detected misalignment. Calibration on the pen cross axis may include setting a scan offset to control the position of a movable carriage mounting the printhead, for example, a b mm offset may be added or removed for a given carriage position, where b is proportional to the detected misalignment.

FIG. 6 represents two other examples of waveforms that may be used to generate interference patterns as described herein. Waveform 610 includes a sawtooth waveform and waveform 620 represents a modified square wave. In practice, other waveforms may be used.

Fig. 7 shows an example 700 of a non-transitory computer-readable storage medium 710 comprising a set of computer-readable instructions 720. The instructions 720 may be executable by a processor 730 of the printing system. The computer-readable storage medium 710 may be the memory of a processor, such as an embedded processor or microprocessor that forms part of a printing system (e.g., forms part of a printer). The memory may include non-volatile memory that stores instructions when not powered and volatile memory that loads instructions during use for execution by the processor.

Instructions 740 cause the processor to instruct printing of the print calibration image using the printing system. The printed calibration image may contain an interference pattern as described herein. The instructions 750 cause the processor to receive captured data from an optical sensor associated with printing a calibration image. Instructions 760 cause the processor to calibrate the printing system based on the captured data. Thus, these instructions may implement the method of FIG. 5 and use the printed calibration image described in the other examples.

As with other examples, the waveform may comprise a continuous sinusoidal waveform having a plurality of periods. In some cases, the print axis comprises a scan axis of a printhead of the printing system, such as direction 130 in fig. 1. In this case, the instructions may cause the processor to determine a location within the scan axis corresponding to a minimum value within the captured data, for example as shown in fig. 3. In some cases, the printing system comprises an inkjet printing system.

The foregoing description has been given to illustrate examples of the principles described. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is to be understood that any feature described in relation to any one example may be used alone, or in combination with other features described, and may also be used in combination with any feature of any other example, or any combination of any other example.

Claims

1. A method of calibrating a printer, comprising:

printing an interference pattern on a printing medium;

capturing data representative of the interference pattern printed on the print medium by using an optical sensor;

calibrating the printer based on the captured data,

wherein the interference pattern is based on a waveform that varies in amplitude along an axis perpendicular to the axis of printing being calibrated, an

Wherein the interference pattern comprises a repeating set of a plurality of patterns based on the waveform, the repeating set having varying pattern spacing on the calibrated print axis.

2. The method of claim 1, wherein the waveform comprises a sinusoidal waveform.

3. The method of claim 1, wherein a field of view of the optical sensor is a non-zero multiple of a period of the waveform.

4. The method of claim 1, wherein printing the interference pattern comprises:

printing a first set of patterns based on the waveform using a first printhead; and

printing a second set of patterns based on the waveform by using a second printhead, wherein the first and second sets of patterns comprise the repeating set of a plurality of patterns based on the waveform, an

Calibrating the printer includes aligning the first and second printheads based on the captured data.

5. The method of claim 1, wherein calibrating the printer comprises:

detecting an extremum in the captured data; and

using the spatial locations corresponding to the extrema to calibrate the printer.

6. The method of claim 1, wherein printing the interference pattern comprises:

printing an interference pattern for a print axis perpendicular to a length axis of the print head; and

the interference pattern is printed for a print axis parallel to the length axis of the print head,

wherein the nozzles of the print head are aligned along a length axis of the print head.

7. A printing system, comprising:

a print head for printing an image on a print medium;

an optical sensor for capturing data from the printed image;

a memory for storing a definition of a printed calibration image comprising interference patterns of varying spacing on a printing axis, the interference patterns comprising waveforms of varying amplitude along an axis perpendicular to the printing axis; and

a calibration controller to calibrate the printing system on the print axis, the calibration controller comprising a processor to:

obtaining a definition of the print calibration image from a memory;

instructing printing of the print calibration image using the print head;

receiving captured data from the optical sensor associated with the printed calibration image; and

processing the captured data to determine calibration parameters for the printing system.

8. The printing system of claim 7, wherein the optical sensor comprises a reflective sensor.

9. The printing system of claim 7, comprising:

a movable carriage in which the printhead is mounted,

wherein the nozzle extends along the print head in a pen axis direction perpendicular to a scanning direction of the movable carriage,

wherein the printing axis is at least one of the pen axis direction and the scanning direction.

10. The printing system of claim 9, wherein the optical sensor is configured to move across the print medium in the scan direction.

11. The printing system of claim 10, wherein the captured data includes measured light intensity values for a plurality of spatial locations across a width of the print medium.

12. A non-transitory computer readable storage medium comprising a set of computer readable instructions stored thereon that, when executed by a processor of a printing system, cause the processor to:

instructing use of the printing system to print a printed calibration image comprising interference patterns of varying spacing on a printing axis, the interference patterns comprising waveforms of varying amplitude along an axis perpendicular to the printing axis;

receiving captured data from an optical sensor associated with the printed calibration image; and

calibrating the printing system based on the captured data.

13. The medium of claim 12, wherein the waveform comprises a continuous sinusoidal waveform having a plurality of cycles.

14. The medium of claim 12, wherein the print axis comprises a scan axis of a printhead of the printing system, and the computer readable instructions cause the processor to:

a location within the scan axis corresponding to a minimum value within the captured data is determined.

15. The media of claim 12, wherein the printing system comprises an inkjet printing system.